Equine Herpesvirus

Overview

Equine Herpesvirus-1 (EHV-1), also known as rhinopneumonitis or “rhino”, is an important cause of disease in our horse population. While there are several types of equine herpesvirus, the focus of this article will be on EHV-1, which can cause a variety of problems including, respiratory disease, abortion, neurologic disease and early foal death. This virus can be highly contagious, and has the unique characteristic of being able to remain dormant (latent) for periods of time and then reemerge during times of stress, such as transportation or showing.

The respiratory form is most commonly seen in young horses (<2 years of age).

The neurologic form is recognized in middle age to older horses.

The virus is passed in the saliva and nasal discharges from horse to horse or through contact with abortion materials (fetus or placenta).

The virus can persist in the environment and often contamination or exposure occurs before clinical signs are noticed allowing the rapid spread of disease and occasionally outbreaks when many horses are comingled.

Signs & Symptoms

The most consistent sign of equine herpesvirus is fever; however, this unfortunately is often missed as signs of a fever are subtle such as mild to moderate depression and decreased appetite.

The respiratory form of EHV-1 may cause nasal discharge, increased respiratory rates, fever and occasionally a cough. Pregnant mares usually do not show any signs of disease prior to abortion; however a history of respiratory disease on the farm prior to the abortions is common.

In the neurologic form of the disease called equine herpesvirus myeloencephalopathy (EHM), horses can display two fever spikes (bi-phasic fever), show signs of weakness, be unable to stand, have a loss of tail tone and develop urine dribbling or become unable to urinate.

Diagnosis

When the clinical signs above are noted or EHV-1 is suspected, a complete physical examination by a veterinarian including a rectal temperature is critical. Your veterinarian may suggest taking a sample of the nasal discharge with a cotton swab and a blood sample to submit to the laboratory for polymerase chain reaction (PCR) tests. This test can identify DNA of the herpesvirus. Additionally, this test can differentiate between the neuropathic and non-neuropathic forms of EHV-1 and give an idea of the amount of virus present which will help with prognosis. Other tests may be recommended by your veterinarian or by a board certified internal medicine specialist to differentiate EHV-1 from other diseases that cause similar signs.

Treatment & Aftercare

If any of the above symptoms are noted, it is critical to begin taking temperatures of all exposed horses. When respiratory signs are noted, usually the virus is allowed to run its course, and as long as your horse is eating and drinking well, no treatment may be necessarily. Occasionally anti-inflammatory drugs may be prescribed. With abortion, it is critical to isolate (segregate) the mare who aborted and remove all fetal tissues to ensure she does not expose other horses to the virus while she is actively shedding. The more complicated situation is that of EHM where the horse is showing neurologic signs. Here the horse may be treated with anti-inflammatory drugs or steroids to decrease the inflammation of the spinal cord. Supportive care including hydration, nutrition, ensuring a safe environment, with or without a sling to support standing and possibly a urinary catheter are utilized. Some research has suggested a role for antiviral medications and your veterinarian may discuss these options.

Prognosis

Horses usually recover uneventfully from the respiratory and abortion forms. However, horses diagnosed with the neurologic form of equine herpesvirus have a more variable prognosis. Some recover, but those that become recumbent for greater than 24 hours can have a grave prognosis and owners may elect euthanasia.

Prevention

Vaccines are available for the respiratory and abortion forms of EHV-1, and it is important to discuss a good vaccination schedule with your veterinarian to protect your horse from these forms of the disease. At this time, there is no vaccine for the neurologic form of EHV-1, so the horse owner should have good biosecurity measures in place. These measure include:

Segregating any new arrivals onto the farm for a minimum of 2 weeks

Taking daily temperatures

Minimizing comingling of horses at shows and events.

Taking your horse’s temperature is the best thing you can do to catch an early fever, and identify sick horses. If your horse has a fever, seek veterinary care and keep any affected horses isolated from others until your veterinarian clears them of disease.